Nutrient-to-energy ratios have attracted wide interest as indices
of dietary quality. They have been used in establishing standards of
suitable quality (e.g., in the design of diets or in nutrition labelling).
They have also been used as indices to be applied when considering
whether the quality or the quantity of the diet is likely to be more
limiting in particular situations (as in considering whether there may
be greater benefit from specific nutrient intervention than from
improvement in total food intake). In either mode of application, the
underlying question is: “If an individual (or population of
individuals) consumes this diet in amounts that will satisfy energy
needs, will the concentration of nutrients also be high enough to
meet his nutrient needs?”

Platt et al. (1) were largely responsible for the introduction of the
ratio of protein energy to total energy (PE ratio) as a convenient and
useful descriptor of one aspect of dietary quality in human nutrition.
To take into account both the quality and concentration of the
protein, they introduced the concept of net dietary protein calories
as a percentage of total calories (NDPCals %).1 In their work it was
applied to the assessment of the adequacy of intakes. For this
purpose, the reference PE ratio was calculated as the simple ratio of
protein requirements (expressed as equivalent energy) to energy
requirements.

In the years that followed, the use of the PE ratio became the topic
of much confusion and controversy. It now seems clear that it is a
useful concept within limited ranges of application but that, since a
reference PE ratio intended to be used as a standard of adequacy can
be calculated in different ways, the proposed application of any
derived reference ratio must be specified clearly and the limitations
to its interpretation must be appreciated. This section attempts to
link the principles and judgements that led to the estimation of
protein and energy requirements in this report to the derivation and
interpretation of reference PE ratios based on these requirement
estimates. Emphasis continues to be placed upon the principles that
should be applied.

The calculation of PE ratios which describe known diets is
straightforward and noncontroversial. Confusion about the PE ratio
relates to the reference criteria to be applied to these calculations and
has arisen from three main sources:

inappropriate calculation of the reference criterion of a safe
or adequate PE ratio as the simple ratio of the recommended or safe
level of protein intake (usually, average requirement + 2 SD) to the
energy requirement (usually average requirement);

failure to recognize the difference between a reference PE ratio
applicable to an individual diet (observed intake of an individual)
and one applicable to the average diet of a group of population (all
individuals not eating the same diet); and

failure to recognize that although energy requirements change
with activity and life-style protein requirements do not, and hence
the criterion of an adequate PE ratio is affected by the physical
activity and life-style (actual or desired) of the individual(s).

This last point emphasizes the fact that reference PE ratios are
situation-specific. Unless such ratios are empirically similar in all
situations of interest (a hypothesis that can be examined through
appropriate calculations), there can be no universal criterion of a
suitable PE ratio (2–6).

In considering the nutritional suitability of the diet consumed by
an individual, the natural reference standard is the ratio of that
individual's protein and energy requirements. As is pointed out in
this report (sections 2 and 11), the true requirement of a random
individual is not known. It can be described only in probability terms
as part of the distribution of requirements in a class of similar
individuals. It is logical, therefore, that the appropriate PE ratio for
an individual should be described as part of the distribution of
possible satisfactory ratios of protein to energy requirements among
similar individuals. This distribution would include all possible
situations in which protein needs would be met when enough of the
diet was ingested to meet energy needs. It would take into account
the situation in which a random individual's protein needs were high
and his energy needs were low as well as when another individual's
protein needs were low and energy needs were high. It would
consider the likelihood of these situations in any class of individuals
who are similar in terms of the variables used to classify individuals
for the description of requirements (section 2). From the distribution
of possible PE ratios, probability statements may be derived
concerning the probable adequacy or inadequacy of any particular
PE ratio for an individual of the specified type (age, sex, activity,
etc.).

Although the distribution of possible satisfactory PE ratios is not
a standard statistical one, an approximate approach to its derivation
can be developed. One such approach is described in Annex 9(A).
Given reasonable estimates of both the average requirements for
protein and energy and the correlation between requirements for
protein and energy within groups of similar individuals, the equation
presented in Annex 9(A) can be used. The equation defines a PE
ratio such that, with any probability (or risk) one wishes to assign,
there is confidence that a higher observed dietary ratio would meet
or exceed the true requirement ratio of a random individual in the
group—a “safe” PE ratio for that class of individual. This particular
statistical and conceptual approach follows from earlier work on the
PE ratio (6) and from application of Fieller's theorem (7).

The concept underlying this approach is shown in Fig. 4. The
curve shown is the distribution for a specified class of individuals of
all possible PE ratios that would meet the criterion of adequate
protein intake when energy needs are met. This can be transposed
into a probability curve describing the likelihood that a given PE
ratio will or will not meet the protein requirements of a randomly
selected individual when enough of the diet is ingested to meet his
or her energy requirement. This concept is entirely analogous to that
described for the distribution of protein requirements (section 2) and
to the interpretation of requirement distributions in terms of
statements of risk (sections 2 and 11). By applying the equation in
Annex 9(A), the PE ratios associated with particular probabilities or
risks can be estimated. By assigning a probability of 0.975 (a “risk”
of 2.5 in 100) one can describe the “safe PE ratio” of the diet in a
manner analogous to the “safe level of protein intake”. However,
in practice, it must be recognized that the ratio is meaningless unless
the energy needs are met by ingestion of enough of the diet; if the
total intake of food is inadequate to meet energy needs, protein
metabolism might be compromised and protein needs would not be
fulfilled at the stipulated PE ratio. In addition, the derivation of the
ratio requires that the situation for which energy needs are estimated
should be defined (see later discussion). These are clear limitations
to interpretation.

Fig. 4. Distribution of required PE ratios

10.2.2 PE ratios applicable to average diets of populations

Further adjustments have to be introduced when deriving
reference PE ratios which can be used to judge the suitability of the
average PE ratio of self-selected diets consumed by a group of
individuals or a population.

Among individuals consuming self-selected diets, it must be
expected that the PE ratio of ingested food varies from one person
to the next. The average PE ratio for the group does not describe
the diet consumed by all members of it. An example of this situation
is shown in Fig. 5. Studies in North America and Guatemala suggest
that the coefficient of variation of the PE ratio of ingested diets
among comparable individuals may be some 10–15% (8,9). The
approach to the development of a reference PE ratio described in the
section above refers to the ratio that should characterize a particular
diet, i.e., the one actually ingested by a particular individual. Clearly
it is inappropriate to use this as a reference ratio in judging the mean
of the distribution of ratios among self-selected diets. One approach
would be to apply the concepts described above to the PE ratio of
the diet for each individual and then to aggregate the probability (or
risk) statements for the whole group (see discussion of this approach
to the interpretation of protein requirements in section 11). Another
approach would be to derive estimates of average requirements for
energy and protein, the variabilities of each, and correlations
between them, at the level of the group or population and then to
apply the concept and approach developed above with the
population as the unit of observation rather than the individual. For
reasons described in section 11, such an aggregation of requirement
estimates is quite complex. The derivation of population level PE
ratios is well beyond the scope of this report.

The more overlap of these distributions there is, the greater will be the expected number
of individuals with inadequate protein intakes, given the condition that all individuals satisfy
their energy needs. A knowledge of both distributions would be required to predict the
prevalence of inadequacy.

The important point that emerges from these considerations is
that “per caput” or average dietary PE ratios must not be compared
with reference ratios calculated by applying the equation given in
Annex 9(A), which is based on information about the requirement
distributions of individuals. A reference PE ratio derived in this way
would be too low, since it does not take into account the variance
of dietary PE ratios (or indeed the variance of reference PE ratios
among classes of individuals). Earlier work, with a more restrictive
model, suggested that the per caput reference PE ratio might be
15–20% higher than that provided by the equation given in Annex
9(A) (4). It now appears that even this may be an underestimate.
Clearly any aggregate comparisons of actual and desirable PE ratios
must be developed and interpreted with great care.

If reference PE ratios are calculated as described above for
individuals or classes of individuals, based on the requirement
estimates presented in this report, it will be found that the requisite
PE ratio rises with increasing age after infancy and early childhood.
The reference ratio for the older adult (60+ years) is likely to be
much higher than that for the preschool child. Although this may
appear to contradict commonly held notions about which age
groups are more vulnerable to “diet quality”, it is a perfectly logical
conclusion from the requirement estimates presented in this report.
The reference PE ratio does not rise because of a progressive increase
in protein need per unit of body weight with increasing age; after
early childhood there is relatively little change. Rather, the ratio rises
because with increasing age there is a progressive fall in estimated
energy needs in proportion to body size. The denominator of the
ratio falls in relation to the numerator.

A similar phenomenon is observed in young adults as physical
activity and hence energy requirements change. For very inactive
individuals the reference PE ratio will be quite high; as activity
increases, the ratio will fall. Accordingly, it should be recognized
that the derived reference PE ratio will be very sensitive to measured
or assumed levels of physical activity (5, 6) and hence to particular
social situations.

In the present report it has been pointed out that energy
requirements may be estimated in terms of the existing body size and
composition and existing patterns of activity (the status quo
approach) or they may be estimated in terms of “desirable”
parameters of body size and activity (the normative approach). It
is logical to assume that for any stable population, there will have
been a variety of accommodations or adaptations that have
established an equilibrium between energy intake and expenditure.
Thus, it also seems logical to assume that existing energy intakes are
adequate or nearly adequate, in most situations, to meet existing
energy needs. It is when a normative approach to growth rate, body
size in adults, and activity profiles for all ages is introduced that
important differences between energy intake and energy requirement
appear.

From the above it follows also that in calculating PE ratios there
are two approaches that may be followed. The status quo approach
would accept existing energy intake as an estimator of existing
energy requirement and use this in the calculation of PE ratio. The
normative approach would accept the energy requirement that
would be associated with a desired state of growth rate, body size,
and activity and use this in calculating the PE ratio. In so far as
growth rate and body size descriptors differ in these two approaches,
estimates of protein requirement as well as energy requirement
would be affected. However, for differences in activity profiles, only
the energy requirement estimates would be influenced.

These two approaches can be expected to yield quite different
estimates of appropriate PE ratios. The status quo approach may
yield ratios some 15–20% higher than the normative approach (4).

In the past decade or so, since the publication of the report of the
Joint FAO/WHO Ad Hoc Expert Committee on Energy and Protein
Requirements (10), it has been argued by many that both energy and
protein intakes are low in comparison to requirement estimates for
low income populations in developing countries. However, it also
has been argued that if actions are taken to increase energy intakes
to approximate requirement estimates, the accompanying increase
in protein intake would meet or surpass requirement estimates (i.e.,
the PE ratio of existing diets is adequate but the total level of food
intake is low). If one accepts the status quo approach to energy intake
and requirement, the argument of relative adequacy may be
fallacious. If existing energy intakes and the associated body sizes
and activity profiles are deemed acceptable, then it is appropriate to
assess protein intake without major regard to energy (or to calculate
PE ratios using existing energy intakes as the estimate of energy
requirement). This could lead to a different conclusion about the
probable adequacy of existing diets, at least for some population
groups.

Although recognizing the practical importance of these
considerations, the Consultation did not attempt an analysis of
dietary data to explore the implications. Rather, this discussion is
presented to emphasize the need for caution in the calculation,
application, and interpretation of PE ratios. The Consultation also
emphasized the need to consider very carefully the goals of
interventions (in terms of growth rates, body sizes, and activity
profiles) in any consideration of the suitability of nutrient: energy
ratios.

10.3.2 Catch-up growth as a special situation

During rapid growth there is a change in both energy and protein
requirements per unit body size. Because these changes differ in
relative magnitude, the appropriate reference PE ratio will change.
The method used to calculate the reference PE ratio does not change,
but the requirement estimates will differ. These changes were
discussed in section 9. The important practical point is that while
reference PE ratios may be expected to rise with increasing assumed
growth rates, there may be practical external limitations on the rate
of catch-up growth that can be achieved by children in the
community; there would be little merit in establishing reference PE
ratios for growth rates that cannot be attained (3, 11). Similarly, if
the quantity of food offered to a child limits the rate of catch-up
growth, a higher concentration of protein in the food may have no
real value. A careful analysis of the situation is needed before
reference PE ratios for catch-up growth can be calculated and
interpreted meaningfully.

To be useful the PE ratio must take into account the digestibility
and amino acid composition of dietary protein. How these affect
protein needs or the efficiency of utilization of dietary protein has
been discussed in section 7. Reference PE ratios could be adjusted
upward to allow for the digestibility and amino acid composition of
diets ingested by particular age groups. This was the method
adopted in the past. However, in many applications a more useful
approach would be to adjust the observed protein intake to a value
that represents the intake of utilizable protein. The reference PE
ratio would then continue to be expressed in terms of protein
equivalent to egg or milk protein and the dietary protein would be
adjusted to this base. This is analogous to the approach of Platt et
al. (1) in their use of the term NDPCals %, although the methods
of calculating the reference PE ratio and adjusting for protein
quality are different in detail.

10.3.4 Other measures of dietary quality

This section has discussed only two parameters of the nutritional
quality of diets, energy and protein, combined in the form of
protein : energy ratios as an index of diet quality. It must be
recognized that the concentration of all other necessary nutrients
should be considered when assessing the adequacy of a particular
diet or establishing standards of nutritional quality for foods and
diets. It is potentially dangerous to consider energy and protein
alone.

There is a special situation where the PE ratio (and other
nutrient : energy ratios) of a diet may appear to be adequate, but
where it can be predicted that insufficient amounts of the diet will
be consumed to satisfy either energy or nutrient needs. This happens
when young children are offered a too bulky diet with a low energy
density (section 7.2). The volume of food to be consumed may
exceed the capacity of the child and hence the PE ratio will be
meaningless. The shortfall between the volume of food that a child
needs to consume to satisfy requirements and the volume that it is
capable of consuming depends on both the energy density of the
food mixture and the pattern of the meals provided. The presumed
limitation is the physical capacity of the gastrointestinal system to
accept food offered at particular times. In this situation the
constraints on food intake may be reduced either by changes in the
feeding pattern or the energy density. It has been suggested that
adding oil to increase the energy density would be a practical
solution in many cases (12–14). If the PE ratio of the original diet
were already low, adding oil would be expected to lower the ratio
even further. Comparison of the resultant PE ratio with reference PE
ratios might in some situations indicate that the addition was
inadvisable. This would be an example of an appropriate use of PE
ratios in assessing the quality of diets. This example also illustrates
that much more than the PE ratio must be taken into account when
assessing the quality of diets in the community.